The present invention relates to an image compression encoding and decoding method and apparatus therefor, and more particularly to an image compression coding and decoding method and apparatus therefor which, using a multilevel compression method, reproduces pictures without degradation of picture quality during a special playback such as a high speed search.
Recently, video information compressing technology has been used in processing a video signal as digital data in digital video apparatus such as digital VCR, HDTV, digital video camera and video phone.
Since known image compression technology has a variable number of compressed coding bits according to picture complexity, a problem has been raised about how to keep constant a compression result in digital VCRs, HDTVs and others which require a constant compression result. Especially, for image compression for a DVCR, since its recording medium is magnetic tape which enables only sequential recording and playback, it has more limitations than that of an ordinary transmission line or disk (CD or hard disk). The most serious problem to be solved is a special playback function (also known as trick play) such as a high speed search.
For a solution, when an image compression method and its tape format are selected, movement of the VCR head (head's scan trace) during recording and playback should be considered. In other words, for special functions such as a high speed search, during compression, each image should be exactly the same size, and a portion of data on a tape should correspond to the image's original screen position and be reproduced regardless of other images.
Generally however, during coding, a complicated portion of an image requires a relatively large amount of bits and a simple portion relatively few bits. Accordingly, when video signals corresponding to one picture or its one segment are to be encoded and recorded in a track having a limited length of magnetic tape, and if the number of encoded bits exceed one track, a conventional digital VCR method records the excess of the encoded bits in a next track or, if the encoded bits reach the number of set bits, stops coding the remaining part which is discarded.
However, the recording method of the excess of the encoded bits in a next track has difficulty in a special playback or high speed search, and the coding stop method deteriorates the quality of a reproduced picture because of partial loss of its picture information.